1. Field of the Invention
The present invention is directed to non-emissive display devices, and more particularly to flexible particle-based display devices.
2. Related Art
An electrophoretic display represents one type of particle-based, non-emissive display device that includes an array of several thousand independently addressable microcells, each microcell containing a small quantity of electrophoretic ink that is held between a pair of opposed, spaced-apart plate-like electrodes. Each microcell is typically several 10s to 100s of microns in size. The electrophoretic ink includes charged pigment particles suspended in a dyed suspension fluid that is maintained in an enclosed cell region between the electrodes. At least one of the two electrodes is transparent so that the state of the ink can be viewed through the transparent electrode. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to the electrode having a polarity opposite that stored by the pigment particles, thereby facilitating changes in the color displayed through the transparent electrode by selectively changing the electrode polarity. For example, applying a first (e.g., high) voltage potential to the transparent electrode and an opposite (e.g., low) voltage potential to the second electrode causes the pigment particles of a 1-particle ink to migrate to the transparent electrode, thus producing a displayed color that is determined by the pigment particles. Reversal of plate polarity causes the pigment particles to migrate to the second electrode, thereby changing the color appearing on the transparent electrode to that of the dyed suspension fluid. Intermediate color density (or shades of grey) may be obtained by varying the electrode polarity (voltage pulse length and amplitude), thus causing different numbers of pigment particles to migrate toward the transparent electrode. When a 2-particle ink is used, oppositely charged ink particles of different colors migrate to the opposite electrodes depending upon the applied polarity. The display may also contain electrophoretic inks with more than two particles as described in U.S. Pat. No. 6,017,584.
Magnetophoretic displays are another type of particle-based display that utilize a magnetophoretic ink including, for example, dark magnetizable particles suspended in a solution of white non-magnetic particles. The magnetizable particles can be, for example, iron or magnetite particles, and the white particles can be titania based particles or other light-scattering particles. The suspension fluid may be clear, and an added surfactant may be added to help maintain a good dispersion. Upon application of a magnetic field gradient (e.g. by a magnetic needle) the magnetic particles move in the direction of a higher magnetic field strength. This is the principle of a MagnaDoodle display (http://www.howstuffworks.com/magna-doodle6.htm)).
Other particle-based (emissive) displays may consist of suspended fluorescent or phosphorescent particles in which the fluorescence or phosphorescence may be activated by, for example, ultraviolet (UV) light.
With the increase in the demand for flexible (e.g., computer) displays or electric paper, there has arisen a need for particle-based (e.g., electrophoretic or magnetophoretic) displays in which the particle-based ink (e.g., electrophoretic or magnetophoretic ink) is reliably sealed between two flexible substrates. Adjacent microcells of flexible electrophoretic displays are typically separated by vertical side walls to prevent settling and agglomeration of the particles, and serve as spacers between the two opposing electrodes. The height of the microcells is typically in the range of about 5 microns to about 200 microns (particularly in the case of magnetophoretic displays the height may be bigger, e.g., in the range of 500 microns—to one millimeter). It is particularly important for flexible electrophoretic displays to provide segmented microcells so that the ink cannot move between the cells. Otherwise the moving liquid would destroy the written image upon bending the display. Furthermore, paths between the cells tend to cause particle concentration gradients.
Conventional methods for producing flexible electrophoretic displays typically include forming five-sided (open-topped) microwells on a flexible base sheet, inserting a small quantity of electrophoretic ink into each microwell, and then forming an upper flexible membrane that attaches to the upper walls of the microwell to seal the electrophoretic ink. The key to successfully producing flexible electrophoretic displays by this method is to form the upper flexible membrane, which is usually liquid (viscous) or at least tacky in the uncured form, without displacing the ink in the open microwells, or causing the adhesive to interact with the ink in an undesirable manner (e.g., contaminating the ink such that agglomeration occurs).
One conventional approach that addresses the problem of forming membranes without displacing the ink is to utilize a thin layer of a relatively low specific gravity adhesive (or, generally speaking, an “uncured polymer”) that floats on a relatively high specific gravity ink. In one version of this method, the ink is inserted in the microwells, the adhesive is dispensed over the filled microcells, and then the adhesive is cured to form a membrane. In a second version, the ink and adhesive are mixed, the mixture is inserted into the microwells and allowed to phase-separate (i.e., the adhesive floats to the top of the ink), and then the adhesive is cured to form a membrane.
A problem with the conventional methods for forming flexible electrophoretic displays is that, by requiring the ink to be relatively heavy (i.e., to have a relatively high specific gravity), the conventional methods require the use of relatively costly or hazardous ink types, such as fluorocarbon or solvent-based inks. That is, the success of these conventional methods depends strongly on the chemical and physical properties of the ink and the sealing layer compound, and as such require the use of high specific gravity ink solvents that are relatively hazardous, thus increasing the risk of injury during both production and after-sale use.
What is needed is a method for reliably producing flexible particle-based display devices that facilitates the beneficial use of low-cost, relatively non-hazardous, low specific gravity inks. What is also needed is a low-cost particle-based display produced by such a method.